Oligomerization processes for the production of linear alpha X olefins are well known. For example, from GB-A-1353873 it is known that C.sub.6 --C.sub.20 linear alpha olefins can be prepared from ethene by oligomerization in the presence of a nickel containing catalyst. Linear olefins, especially linear alpha olefins, over a range of carbon chain lengths have found use as valuable intermediates in the preparation of polyolefins, detergents and lubricant additives. While the known processes for the oligomerization of ethene provide linear olefins of which the number of carbon atoms is even, for certain application areas of olefins, it is desirable to have olefins which have special structural features, such as linear alpha olefins which have an odd number of carbon atoms or alpha olefins which have one or more hydrocarbyl substituents located at or near the end of the carbon chain opposite to the end at which the double bond is located. The object of the present invention is the provision of an oligomerization process by which olefins having such special structural features can conveniently be prepared.
A process for the preparation of olefins of the general formula R--(CH.sub.2 --CH.sub.2).sub.p --CH.dbd.CH.sub.2 in which formula R is a hydrocarbyl group and p is an integer of at least 1, comprising reacting ethene with at least one further alpha olefin of the general formula R--CH.dbd.CH.sub.2 under oligomerization conditions in the presence of a catalyst system obtainable by combining a first component which is a bis(cyclopentadienyl) Group IVA metal compound containing a substituent which is attached to the metal and which is capable of reacting with a cation, and a second component which is a compound having a bulky anion containing a plurality of boron atoms and which is substantially non-coordinating under the reaction conditions and a cation, and recovering an oligomeric product comprising linear olefins of the general formula R--(CH.sub.2 --CH.sub.2).sub.p --CH.dbd.CH.sub.2, is the subject matter of European Patent No. 91200367.0. By this co-oligomerization process it is possible to prepare mixtures comprising co-oligomers of high linearity which may have an odd or an even number of carbon atoms and which have a high content of alpha olefins. These oligomeric products are prepared in conjunction with oligomers of ethene from which they have to be separated in order to obtain them in a state of high purity. This separation may in some instances be difficult to perform.
It has now been found that, when in this process the further alpha olefin, designated by the general formula R--CH.dbd.CH.sub.2, in which R is defined as hereinbefore, is present in the reaction mixture in a quantity of at least 1 mole per mole of ethene and at least one of the cyclopentadienyls of the metal compound is (cyclo)alkyl and/or arylalkyl substituted, an oligomerization product can be obtained which has a high content of olefins of the general formula R--(CH.sub.2 --CH.sub.2).sub.p --CH.dbd.CH.sub.2, in which R and p have the meanings given hereinbefore. The oligomerization product thus obtained has a very low content of other oligomers, for example oligomers of ethene and co-oligomers of ethene and the further alpha olefin, such as co-oligomers which have the double bond in the 2-position.
As products of the general formula R--(CH.sub.2 --CH.sub.2).sub.p --CH.dbd.CH.sub.2 may comprise linear alpha olefins which have an odd number of carbon atoms or alpha olefins which have one or more hydrocarbyl substituents located at or near the end of the carbon chain opposite to the double bond, the present process is capable of providing olefins with special structural features as indicated hereinbefore, in a single step from readily available feedstock. The desired oligomeric alpha-olefins can be prepared with relatively little formation of by-products and, accordingly, they can be purified relatively easily. The present process therefore constitutes an improvement over the process described in European Patent No. 91200367.0.